West Nile virus was first detected in North America in 1999 and has spread throughout the United States and Canada and into Mexico and the Caribbean. The cases of encephalitis in New York were diagnosed as Saint Louis encephalitis which was endemic in the North America. However, dead crows and dead flamingo were found around the same time in New York. American birds seldom die by Saint Louis encephalitis virus infection. According to viral isolation and sequencing of the genome, the causative agent of unknown encephalitis was West Nile virus which was a member of Flaviviridae which cause fever, meningitis and encephalitis. West Nile virus is still active in North America in summer season, 1999 -2005. CDC enlightens people how to protect themselves and their community from West Nile virus. There are some reports that West Nile viral infections were occurred by blood transfusion, biologic products from blood and organ transfusion.